Dear SPMers,
We have recently run into an interesting 'feature' of our MR scanner
that produces a small, but significant artifact in fMRI time series. We
are curious to know if anyone else has observed this artifact or has
come up with other ways to deal with it other than that listed below.
It is my understanding that, by default (at least on GE scanners), rf is
applied in an alternating fashion in EPI single shot acquisitions. This
is known as rf-chop on our machine and it is typically used to increase
SNR when combining acquisitions as in typical MR applications. However,
we have observed that this feature produces stronger signal when rf is
applied in one direction than when it is applied in the other direction.
Moreover, these effects seem to be vary in different slices within
acquisitions. Thus, there is a large amount of image intensity variance
between acquisitions. The tell tale sign of this artifact is the
presence of a small 'saw tooth' like (about .2 mm) movement estimate in
the realignment output.
We have recently been able to compensate for this feature by simply
adding a covariate of no interest or 1 -1 1 -1 1 -1......etc for each
image in the time series during statistical analyses. Of course, we do
not convolve this with covariate of no interest with the hrf. This
effectively removes the variance associated with this artifact from our
data. Indeed, z-scores typically jump from the 5 to 5.5 range to over
6-7.0 range when controlling for this artifact.
I would very much appreciate any comments that any other users,
physicists in particular, might have on this issue.
I discussed this issue with members from several other fMRI labs that
have observed this artifact in their realignment time series at the
recent Cognitive Neuroscience meeting in Washington. They had not been
sure what to make of it. I hope that this note helps others as well.
Best regards,
Kent Kiehl
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Kent A. Kiehl
2255 Wesbrook Mall
Departments of Psychiatry and Psychology
University of British Columbia
Vancouver, B.C., Canada V6T 2A1
work (604)822-7128
home (604)221-7809
fax (604)822-7756
email [log in to unmask]
http://www.psychiatry.ubc.ca/sz/nilab/kiehl
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